1. Field of the Invention
This invention generally relates to the maintenance of data redundancy in data storage facilities and more particularly to recovering data in such a facility.
2. Description of Related Art
Many data processing applications now operate on digital data processing systems that incorporate large scale memory facilities. These large scale memories take many forms. One such form is a disk array storage device (DASD) in which data is stored at a local large scale disk memory with many physical disk drives. In other applications it may be desired to distribute the functions of equivalent devices over a network at different network nodes.
When maintaining data integrity is critical, such large scale disk memories include some type of data redundancy. Memory mirrors provide on-site redundancy to protect against equipment failure. U.S. Pat. No. 5,206,939 to Yanai et al. and assigned to the same assignee as this invention discloses a remote system that normally acts as a mirror of a local system. The geographical separation between the local and remote systems provides an extremely high degree of reliability, particularly as protection against a natural disaster.
More recently U.S. Pat. No. 6,101,497 granted Aug. 8, 2000 discloses a tool that is particularly well adapted for facilitating redundancy. This device, called a BCV device, establishes a special physical disk structure that acts as a moving mirror to attach to and to synchronize with a standard or production volume or device without interfering with normal operations between an application program being run by a host. Once this BCV moving mirror is synchronized with the standard volume, it can be split, or isolated from the standard volume and thereafter be available for backup or for use by another application.
As known, data on a standard volume can, for a variety of reasons, become corrupt. As described in the foregoing U.S. Pat. No. 6,101,497, a restore or an incremental restore command initiates a process by which data in the redundant BCV physical mirror is replicated to the standard or production volume, assuming that data on the BCV physical mirror has not been changed as, for example, if the BCV physical mirror when isolated, is a source for a tape backup. The data on the BCV physical mirror represents the data that existed when the standard volume and the BCV physical mirror were isolated from each other. However, many databases and other applications maintain a log file, or like file that records each transfer and the sequence of each transfer. Entries from such a file from a point in time after the prior isolation can be used to recover the data fully on the standard volume. This process is known as “unrolling the redo logs”.
During this process, the BCV device is attached to the standard volume. Consequently, as each log entry is processed, a resultant write operation transfers the data associated with that log entry to both the standard volume and the BCV physical mirror. Consequently, after the first write operation occurs, the data on the BCV physical mirror is no longer identical to the data that had been on the BCV physical mirror at the time of a prior split. Oftentimes errors can occur corrupting the data on the standard volume and, due to the mirroring function, the data on the BCV physical mirror. When this occurs it may be required to recover data from a tape backup made even earlier in time and again try to reconstruct the current data by unrolling the redo logs for a longer time interval. Data recovery involving tape backups can be time consuming and tedious.